In longwall mining, the breast or face of a coal seam, for example, can be mined by displacing along the breast of the seam, a row of conveyor elements which can be linked together and can be provided with a cutter, e.g. a coal plow (see U.S. Pat. No. 4,048,804, for example) to excavate the coal into the conveyor.
The conveyor generally comprises flights joined together by an endless chain and movable along a trough to carry the mined material to a tunnel or drift formed along a side of the path of the mining apparatus and thus along the coal seam to be excavated so that the coal can be brought out of the mine.
The mining machine can comprise a prop connected to each of the conveyor elements or segments by a displacement cylinder and can have one or more rams adapted to press a cap against a roof of the chamber in which mining is effected and usually overhanging the conveyor.
Behind the machine in the goaf, the roof can be permitted to collapse. The advance of the machine is effected in a stepwise manner with the displacement cylinders being actuated to advance the conveyor elements, the cutter and the mining front into the breast of the coal seam with the props braced between the roof and floor and then, by contraction of the displacement cylinders or retraction of the rods of these cylinders, with the ram pressure relieved, the props are drawn forwardly behind the conveyor segments.
The control of the movement of the assemblies defining the excavation front is important in mining, because the geological formations are generally not homogeneous so that it is not possible to advance the cutting front and the respective conveyor elements at the same rate in a perfectly straight line perpendicular to the advance direction against the breast of the longwall of the seam which is to be excavated and to successfully maintain the advance with the lines of conveyor elements always parallel to one another and straight.
The control of the props has generally been effected by individual controls for the various props utilizing one of a number of standard control techniques. The control systems which have been used include individual control, sequencing control, group sequence control and central control.
Of these control techniques, central control is the most automated. It is known to provide this type of control to establish a line for the props which corresponds to a setpoint line for the excavation with respect to which deviations are determined which can arise because of the differences in the hardness of the minerals to be excavated by the respective assemblies along the mining front.
In the past and in practice, the detected deviations were corrected by manual control of the advance of the respective assemblies in a time-consuming and labor-intensive manner.
German patent document No. 15 33 720 describes a process for controlling the excavation front in which the prop structures disposed within a mine tunnel and arrayed next to one another along a longwall face are associated with guides individually or for each group of props. Each of the prop structures can have a measuring device for detecting the number of steps and the respective step widths. The measurement data are supplied to a central computer which determines the difference between the step widths between the guide prop structures and the usual prop structures, compares these differences with a threshold value and upon exceeding of the threshold, commands a signal for actuating the displacement cylinder or for corresponding control of the excavating device.
A drawback of this process is that it requires intervention in control since the guide prop structure does not have any displacement measuring device and thus cannot be provided in a feedback path for control. Position measurements are derived from displacement differences between the advanced prop and the guide and it is thus difficult, if not impossible, to ensure excavation, corresponding to predetermined setpoint lines.
Problems in the region of the guide prop structures, whether resulting from displacement of the conveyor or by the drawing of the props after them, cannot be taken into consideration in this process and can result in an adverse effect on the entire mining process over the whole mining front. It is especially necessary to have manual intervention when it is necessary to swing the excavation front and it is frequently necessary in these cases to interrupt the mining which is uneconomical.
When a system is provided which operates purely with actual value-position detection, passive influences on the control system can be observed, i.e. the point in time or state at which a control action must be taken, may not depend upon actual effects, but rather on phenomena that are a consequence of relatively passive elements because of the need to exceed certain thresholds or the like. Thus the history of a particular state plays a significant role and excessive deviations may not be sufficiently corrected and can pyramid into problem conditions. In conventional systems for controlling the movement and deviation of the mining front, therefore, local deviations may have to be permitted and can develop into problems.